If you download Windows 7 via MSDN, there is a note when you retrieve your Windows 7 Beta key, which reads:
Product keys made available for the Windows 7 Beta program are for the Windows 7 Beta Ultimate edition which is the only real beta edition being made available. Please install Windows 7 Beta Ultimate edition and use the Ultimate edition with the product key provided. The SKUs you are seeing in the checked build are legacy SKUs of the Windows Vista SKU plan, and do not represent the Windows 7 SKU plan. We recommend and encourage you to focus testing and development on the Ultimate edition only.
To downgrade the human mind is bad theology. - C. K. Chesterton